The present invention relates to a composition of an expandable styrene polymer in the form of beads, to a process for preparing the composition and to the materials manufactured from the composition, such as expanded beads and moulded parts of expanded polystyrene.
Expanded polystyrene is an example of a low-density, rigid, cellular material whose main uses are for the thermal insulation of buildings and for the packaging of food or industrial products.
The most remarkable properties of expanded polystyrene are its low bulk density, which may range from 7 to 50 kg/m.sup.3, its very low thermal conductivity, its good ageing resistance and its good behaviour in water. Its mechanical and thermal properties firstly depend on the bulk density of the material and also on its cellular structure, i.e. on the diameter of the cells, on their distribution and on the corresponding wall thickness.
Expanded polystyrene is especially obtained from polystyrene in which a blowing agent such as pentane has been dissolved this is what is called expandable polystyrene, which is in the form of beads and is the raw material used for moulding parts or articles from expanded polystyrene.
Expandable polystyrene may be obtained either in two steps, comprising polymerization in aqueous suspension of styrene followed by impregnation of the polystyrene by a blowing agent, or in a single step in which the polymerization and impregnation are carried out simultaneously.
The production of expanded polystyrene from beads of expandable polystyrene usually includes three steps:
(a) the beads of expandable polystyrene are firstly expanded by mixing them with steam in a stirred tank: this is the pre-expansion or prefoaming in which the volume of the beads may be increased by a factor of 50 or 60; PA1 (b) the expanded beads are then stored for a few hours in the open air: this step corresponds to stabilization or maturing of the beads; PA1 (c) the beads thus expanded and stabilized are then welded together in moulds by a new heating operation: this is the moulding operation. PA1 from 2 to less than 5 parts by weight of the blowing agent; and PA1 from 0.2 to 0.9 part, preferably from 0.3 to 0.6 part, by weight of the petroleum wax. PA1 from 5 to 9 parts by weight of the blowing agent; and PA1 from 0.1 to 0.6 part, preferably from 0.1 to less than 0.5 part, in particular from more than 0.1 to 0.45 part by weight of the petroleum wax. PA1 from 3 to less than 6.5 parts by weight of the blowing agent; and PA1 from 0.2 to 0.9 part, preferably from 0.3 to 0.6 part, by weight of the petroleum wax. PA1 from 6.5 to 10.5 parts by weight of the blowing agent; and PA1 from 0.1 to 0.6 part, preferably from 0.1 to less than 0.5 part, in particular from more than 0.1 to 0.45 part, by weight of the petroleum wax. PA1 Content of mineral hydrocarbons with carbon number less than 25: not more than 5% (w/w): PA1 Dynamic viscosity not less than 11 mm.sup.2 /s at 100.degree. C. (ASTM D 445); PA1 Weight-average molecular mass not less than 500.
Over many years, the properties of expanded polystyrene have been very considerably improved, especially in the moulding step. Thus, it has often been sought to increase the moulding production rate by the choice of additives and of surface treatments of the beads. Products have thus been obtained which weld together well and can be more rapidly demoulded, especially because of a shorter time for cooling the moulded material.
However, little effort has been put in to improving the pre-expansion or prefoaming step, for example in order to increase the production rate of this step, especially by increasing the rate of expansion of the beads. It is precisely during this step that the bulk density of the final material is essentially determined. Thus, any increase in the expansion rate should preferably be achieved while maintaining the uniform development of the beads without affecting their cellular structure (cell sizes and distributions) or their dimensional stability, nor reducing the moulding production rate in the last step.
Moreover, in recent years it has been sought to greatly reduce the residual monomer content in expandable polystyrene. However, the presence of residual monomer may be seen as a factor promoting the rate of expansion of the beads. Thus, greatly reducing the residual monomer content may be regarded as an additional obstacle to improving the pre-expansion or prefoaming, especially to increasing the rate of expansion of the beads.
One of the objects of the present invention is therefore to find a means or an additive allowing the rate of expansion of the beads to be increased during the step of pre-expanding or prefoaming an expandable polystyrene having a low residual monomer content without, however, affecting the cellular structure of the expanded materials or reducing the moulding production rate in the final step.
British Patent Application GB-1,012,277 describes a process for improving the moulding of expanded polystyrene beads, especially for reducing the time for cooling the moulded material. The process consists in treating the beads of an expandable polystyrene using 0.1 to 10% by weight of a wax, essentially by coating. The treatment of the beads can be carried out only before or after the end of the polymerization, that is to say at the earliest at the time when the beads have formed during the suspension polymerization. This may, for example, take place during the final steps of the styrene suspension polymerization process, under conditions such that the wax incorporated into the polystyrene can migrate to the surface of the beads and thus form a coating sufficient to allow more rapid cooling of the moulded material during the final moulding step. The patent application cites a large number of natural or synthetic waxes, having a melting point of 50 to 85.degree. C. for example, and recommends coating the beads using a synthetic wax, more particularly a polyethylene wax having a melting point of approximately 70.degree. C., in an amount of 1 to 2% by weight with respect to the polymer. According to the patent application, in all cases this is a process which is essentially equivalent to a surface treatment of the beads. It is precisely because of this surface treatment effect that the expanded beads may be welded together more rapidly. A treatment limited to the surface could in no case have an appreciable effect on the expansion of the beads during the prior pre-expansion or prefoaming step since the expansion essentially depends on the uniform and homogeneous character of the structure of the beads.
U.S. Pat. No. 3,060,138 discloses the manufacture of beads of expandable polystyrene, using isopentane as the blowing agent. In order to improve control of the size of the cells in the expanded materials, a C.sub.16 to C.sub.46 hydrocarbon paraffin is dissolved in isopentane in an amount of 0.5 to 2% by weight with respect to the polystyrene. According to this patent, the use of isopentane is essential for reducing the size of the cells, and it is because of a synergy between the isopentane and this paraffin that it is possible to have an effect on the cellular structure of the expanded material and that, under these conditions, the paraffin thus acts as a nucleating agent. No effect on the rate of expansion of the beads during the pre-expansion or prefoaming step is mentioned.
British Patent Application GB-1,289,466 discloses a process for increasing the rate of moulding of expanded polystyrene, especially for reducing the time for cooling the moulded material. The process consists, after polymerization, in coating beads of expandable polystyrene using an oil or a wax or a mixture of an oil and a wax. The patent application cites many natural or synthetic waxes, and more particularly Example 1 shows a paraffin wax used in an amount of 0.01% by weight with respect to the polymer: this is a crude paraffin wax containing 25% by weight of mineral oil. Coating the beads with this paraffin wax allows the moulding to be improved, especially by allowing the moulded material to be cooled more rapidly. The surface treatment effect produced by the coating can in no way provide a uniform and homogeneous character capable of improving the expansion of the beads and especially of increasing the production rate of the pre-expansion or prefoaming step.
U.S. Pat. No. 4,243,717 discloses a process for increasing the rate of moulding of expanded polystyrene, especially for reducing the time for cooling the moulded material, and simultaneously for controlling the cellular structure of the expanded material. The process consists in adding, during polymerization of the styrene, a synthetic wax called Fischer-Tropsch wax having a congealing point of 86.degree. C. to 110.degree. C. and consequently a melting point greater than these values. This Fischer-Tropsch wax used as a nucleating agent is different from petroleum waxes, which in particular have lower molecular masses.
American Patent U.S. Pat. No. 3,224,984 discloses a process for reducing the size of the cells of expanded polystyrene and simultaneously increasing the moulding rate by more rapid cooling. The process consists in adding, during the polymerization of styrene, a synthetic wax, more specifically a polyolefin wax, for example a polyethylene wax having a density ranging from 0.918 to 0.960, or a polypropylene wax. The polyolefin wax is used as a nucleating agent and generally has a weight-average molecular mass of 1000 to 4000. It is different from petroleum waxes, which in particular have lower molecular masses.
European Patent Application EP-0,409,694 discloses a process for improving the productivity of expanded-polystyrene moulding plants, especially by increasing the moulding rate by more rapid cooling of the moulded materials. The process consists in carrying out a styrene suspension polymerization in the presence of 0.01 to 0.6% by weight (with respect to the monomer) of a synthetic wax chosen from polyethylene waxes having a number-average molecular mass of 500 to 5000, or having a softening point of 90 to 110.degree. C. and consequently a melting point greater than these values, or else a melt flow index (ASTM D 1238 standard, condition E) of greater than 100 dg/min. These polyethylene waxes are different from petroleum waxes which, for example, have lower molecular masses.
U.S. Pat. No. 3,647,723 discloses a process for improving the step of pre-expanding or prefoaming beads of expandable polystyrene, especially by reducing the tendency of the beads to stick to one another. The process consists in adding, during the suspension polymerization of styrene, a wax having a melting point of 70 to 120.degree. C., an acid number of 0 to 45 and a saponification number of 3 to 150. This is a wax essentially consisting of esters of alcohols and of aliphatic acids with a long chain--very different from a petroleum wax. The patent cites in Comparative Example B the use of 0.1% by weight of a paraffin wax having a melting point of 58 to 60.degree. C. The wax is introduced during the polymerization at the moment when the degree of conversion of styrene into polymer is 40%. The composition thus obtained has the drawback of being in the form of beads which stick together and disturb the pre-expansion or prefoaming. No mention is made as regards any possible effect on the rate of expansion of the beads.
British Patent Application GB-2,110,217 discloses a method of making expandable polystyrene beads containing 0.05% to 0.5%, by weight based on the styrene, of a polyethylene wax, which is substantially linear and has a density of at least 15.4 kg/m3, a molecular weight of 700 to 1500, a polydispersity of less than 1.2, and a melting point of at least 102.degree. C. This polyethylene wax is different from petroleum waxes.